Fluid particles also collide with squirmers or bounding walls during the streaming step.
In contrast to the
event-driven implementation of
squirmer dynamics,
one
may
not want to calculate exact collision times between fluid particles and squirmers or walls
since this slows down the simulation.
One way to overcome this is to first stream
all
the squirmers
during time \(\delta t\)  neglecting that some
fluid particles may enter
them and only then move the fluid particles.
Padding et al. [1]} have proposed an efficient method to do so for simulating colloidal
suspensions, which has been
used for squirmer-fluid interactions as well [2]}, [3]}, [4]}, [5]}.
We start with a fluid particle hitting a wall. Once this happens,
it is moved half a time step back and its velocity is updated
as follows.
In order to fulfill the no-slip boundary condition at the wall, the bounce back rule is applied,
where the velocity is simply reversed,
\(\mathbf {v}_i^{\prime } = - \mathbf {v}_i \, ,\) 
